Fraser,
I have a small 12Volt panel (10 watt), which I use for keeping my tractor battery topped up. I checked its characteristics by measuring the cell voltage with varying loads on the cell. At no load, the cell voltage was about 21 volts. As the load increased, the cell voltage reduced. For each step of load current, I calculated the cell power by multiplying the load current by the cell voltage. As the load increased, the cell power increased. However, to charge a 12 volt battery, you need about 14 volts. With the cell, I found that the maximum power from the cell occurred at a cell voltage of 14 volts. This was done in bright sunlight. It seems that solar panels for charging 12 volt batteries, are designed so that they can be directly connected to a battery without the need for an extra regulator. Of course, in my case the application is to keep the battery topped up while the tractor is not in use; whereas you are looking for a more serious battery charging setup. To effectively charge 12 volt batteries, you really need to select the cell and the regulator.